Vulcanized rubber is one of the most voluminous and serious waste product sources in the United States. For example, in 1990 approximately 3 billion waste tires existed in the United States alone, and about 250,000,000 more tires are added every year. This makes waste tires one of the most significant environmental and waste storage problems. In addition to environmental concerns, the waste tires pose significant fire and safety problems for those that live in the vicinity of these stockpiles.
Currently, less than 13% of the waste tires generated annually are recycled by any means. The principal methods now in use are tire derived fuel (TDF), pyrolysis for recovering starting materials, rubber modified asphalt pavements and reduction by grinding and cryofracturing for reincorporation into low-performance moldable products (gaskets, athletic track surfaces). An example of a process combining devulcanized rubber with a thermoplastic is U.S. Pat. No. 5,359,007 to Luiz C. Olivera Da Cunha Lima.
Because vulcanized rubber is chemically stable, it is difficult to combine it into new material because bonding between the new material and the vulcanized rubber is weak. Present methods of devulcanization are costly, hence there is a need for a cost effective de-vulcanization process.
Currently, only about 3 to 4 percent by weight of reclaimed rubber can be used in the manufacture of new tires. In this process, the waste tires are ground into a powder, then rinsed (sometimes with hazardous chemicals) and dried, and then mixed within a virgin rubber matrix. The limitation of this process is the low percentage by weight of waste rubber that can be re-used.
Vulcanization
Useful rubber products cannot be manufactured without vulcanization. Unvulcanized rubber is generally weak and undergoes permanent deformation after a large strain is imparted on the article. Conversely, vulcanized rubber forcibly retracts to approximately original shape after a large mechanically imposed deformation. Vulcanization can be defined as a process which increases the retractile force and reduces the amount of permanent deformation remaining after the removal of the deforming force (Mark 1994) and consists of using sulfur compounds to bind the rubber molecules together.
Vulcanization is more particularly described as the process of chemically producing network junctures by the insertion of crosslinks between polymer chains. Sulfur is one of the most common crosslink agents used in the manufacture of tires. The addition of a small amount of sulfur (typically 1-2.5%) in a tire rubber compound is responsible for the majority of the physical properties of the final product, specifically, strength, elasticity, resistance to degradation, and durability. However, these benefits prove to be a major problem when attempting to recycle or re-use vulcanized rubber. The strength of the cross-link bonds created by the vulcanization process prevents the vulcanized rubber from melting, binding within a virgin rubber matrix, or dissolving in solvents.
Hence, there is a need for a method of devulcanization that would permit a substantially greater portion of scrap or waste vulcanized rubber to be re-used.